Etch stop layers are frequently used in semiconductor industry. Siloxane materials are etched by plasma of fluorocarbon gases such as HCF3 and CF4. Siloxane antireflective coatings are frequently used in the industry to facilitate sub-50 nm line widths. A common approach is to have a layer of high carbon content material underneath the Si-ARC to control or stop etching by fluorocarbon plasma. A material well suited for such layers is called amorphous carbon layer (frequently abbreviated ACL, cf. reference 1).
However, since ACL is deposited by CVD, high cost is commonly associated with ACL. Spin-on materials, on the other hand, are known to require low capital investments in tools, which makes them attractive alternatives. However, for being a viable substitute for ACL, the spin-on material needs to fulfill several requirements, such as high carbon content and high thermal stability up to 400° C., often in an oxidative atmosphere. Of course, they need to be soluble in a safe solvent commonly used in semiconductor industry. The film needs to be curable by heat to gain solvent resistance. It also needs to have good adhesion to the layers below and above. The solution on the other hand, needs to have satisfactory storage stability.
Novolacs are materials known for their relatively good thermal stability and good adhesion, and they are widely used in semiconductor industry, for example as dielectric materials and photoresists. Novolacs are polymers that are prepared by reacting phenol with formaldehyde with molar ratio of formaldehyde to phenol typically less than one, using acid catalysts. However, more broadly novolac is considered to be any polymer derived from acid or base catalyzed condensation of aldehydes with phenols or ketone-derived phenolic compounds, such as bisphenol A, which is a condensation product between acetone and phenol.
Aliphatic hydrogen atoms, that is, H-atoms that are covalently bonded to aliphatic C, are known to decrease thermal stability of polymers. This is because aliphatic hydrogen is easier cleaved by heat compared to their aromatic counterparts, yielding to a C. radical, which is susceptible to radical scission depolymerization of the polymer back bone into volatile components.
Scheme 1 shows the thermal stability of aliphatic versus aromatic hydrogen:
So, in theory, removal of aliphatic hydrogens from the polymer backbone should increase oxidative thermal stability of the polymer. However, polymers with high aromatic content tend to have poor solubility in common solvents, and for example poly-p-phenylene, consisting of benzene rings attached to each other at 1 and 4 position to form a chain, has no solubility in common solvents. This makes it unsuitable for polymer solutions for spin-coating applications.